Add oslotest to test-requirements.txt

when executing 'tox -e debug', get error
"could not find executable 'oslo_debug_helper'"

Change-Id: Ic89fcbdc8e97c4da7587ce31fcba12b3f14c474f
Closes-Bug: #1676438
diff --git a/test-requirements.txt b/test-requirements.txt
index 2bf9f2e..74109e4 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -7,6 +7,7 @@
 python-subunit>=0.0.18 # Apache-2.0/BSD
 sphinx>=1.2.1,!=1.3b1,<1.4 # BSD
 oslosphinx>=4.7.0 # Apache-2.0
+oslotest>=1.10.0  # Apache-2.0
 testrepository>=0.0.18  # Apache-2.0/BSD
 testscenarios>=0.4  # Apache-2.0/BSD
 testtools>=1.4.0 # MIT